In recent years, a metamaterial technique of controlling propagation characteristics of an electromagnetic wave by periodically disposing a conductive pattern having a specific structure has been proposed. In particular, a technique of “a frequency selective plate (or a frequency selective surface)” of controlling transmittance characteristics and reflection characteristics of an incident electromagnetic wave by periodically disposing a specific conductive pattern as a minimum unit cell in a two-dimensional manner is paid attention to. A frequency selective plate is also referred to as a frequency selective sheet/surface (FSS), and application thereof to a reflection plate of an antenna and a radome of a radar device is expected.
PTL 1 describes an antenna device including a frequency selective plate in which a ring-shaped conductive pattern is periodically disposed, on a back surface of an antenna. The frequency selective plate described in PTL 1 reflects an electromagnetic wave only at an antenna operating frequency, and transmits an electromagnetic wave of another frequency band. Therefore, it is possible to reduce a radar cross-section (RCS) in a band other than the antenna operating frequency.
Further, PTL 2 discloses a configuration including a frequency selective plate in which metal pieces are periodically disposed, on a front surface of an array antenna, or a configuration in which holes are periodically formed in a metal plate. The frequency selective plate described in PTL 2 is able to transmit an electromagnetic wave only at an antenna operating frequency, reflect an electromagnetic wave of another frequency band, and therefore reduce radiation of an unwanted frequency from an array antenna.